1. Field of the Invention
This invention relates to an aqueous resin composition for processing fibrous sheets which presents no problem with regard to safety, hygiene or pollution, and to its use as the back coating of a separable fastener. A method of manufacturing a separable fastener having excellent characteristics using this composition is also a part of the invention.
2. Discussion of the Background
Separable fasteners known in the art typically comprise two types of sheets, i.e., one sheet having plural hook-shaped catches or mushroom-shaped projections (referred to hereafter as hooks) on the base fabric and the other sheet having plural loop-shaped connectors (referred to hereafter as loops) on the other base fabric, the two sheets being joined by the interconnection of the hooks and the loops. Other fasteners are known wherein hooks and loops are present on the same surface. These separable fasteners are widely used as closure devices.
To manufacture the separable fastener, the hooks (or the filaments for hooks) and the loops are interwoven in a fabric (sheet), an organic solvent solution or aqueous dispersion of a resin is coated on the back surface of the base fabric of the loops or hooks, i.e., the back surface of the base material of the separable fastener, to prevent them falling out due to repeated fastening and unfastening, and the separable fastener is then attached to cloth (woven or unwoven), leather or a plastic sheet such as PVC by sewing or sticking with adhesive, or by high frequency adhesion.
The coating resin was usually a resin such as polyurethane, polyester or polyamide, polyurethane resin being the most widely used due to its high performance.
The polyurethane resin used as the back coating of the separable fastener was in the form of an organic solvent solution or an aqueous dispersion. In the case of an organic solvent solution, there were major problems regarding safety, hygiene and pollution. On the other hand, in an aqueous dispersion, the hydrophilic nature of the polyurethane resin is much stronger than in an organic solvent solution, and the performance of the separable fastener is considerably impaired. For this reason, considering the performance of the separable fastener, organic solvent solutions of polyurethane resin are still being used. In recent years, however, environmental pollution has become a major issue, and a manufacturing method which did not use an organic solvent was much desired.
For example, in Japanese Patent Laid-Open 62-112504, a method is disclosed for manufacturing a separable fastener using an aqueous dispersion of a polyurethane resin wherein the washability and dry cleaning properties, which were formerly problems with aqueous dispersions, are improved. However, in the invention of this disclosure, there is no mention of improvement of deterioration due to chlorine bleaching agents used for separable fasteners on account of their recent, wide use in medical applications in hospitals, or seat covers in automobiles, trains and aeroplanes. The chlorine bleaching agent is usually an aqueous solution of sodium hypochlorite, but as polyurethane resin has a highly hydrophilic nature in aqueous dispersion, it is easily degraded by chlorine bleaching agents. Consequently, the separable fastener obtained using the aqueous dispersion of polyurethane resin in this disclosure as a back coating tends to become ruffled due to the chlorine bleaching process, and its durability is considerably impaired. In the disclosure cited, the improvement of ruffling due to fastening and unfastening after washing (washability) is mentioned. However, better characteristics are now being required concerning also the dropout of fibers from the edges of the separable fastener when it is subjected to stirring in a washing machine, and there is no mention of any method to improve this point. Still further, in the case of separable fasteners having an aqueous dispersion polyurethane resin as a back coating, the phenomenon of crease whitening, wherein a part of the separable fastener which becomes creased shows a white line, tends to occur more easily than in the case of a solvent system, and there is no measure proposed to deal with this problem in the disclosure cited. For this reason, polyurethane organic solvent solutions are still being used for the back coating agent. However, in recent years, attention has been focused on environmental pollution. If back coating agents based on organic solvents are used for the fasteners, the organic solvents are necessarily released into the atmosphere by evaporation and diffusion, and this leads to environmental pollution. It was thus much desired to manufacture the fasteners without using organic solvents.
In the case of fibrous sheets other than separable fasteners, it is common practice to impregnate or coat the sheet with a resin to impart physical properties, improve appearance or confer functionality, and here too, the avoidance of organic solvents was much desired to preserve the environment and maintain safety.